mail/mu: Update to 1.12.13 and take maintainership

Changelog: https://github.com/djcb/mu/blob/v1.12.13/NEWS.org

PR: 289593
Reported by: RwN <rwn@mailo.com> (new maintainer)
Tested by: Trenton Schulz <trueos@norwegianrockcat.com>
Approved by: hrs (previous maintainer, timeout >5 weeks)